Conventionally, a melting facility in a steel mill that accepts scrap iron and steel as an input and produces molten steel ready for casting in a continuous caster as an output (frequently referred to in the industry as a "melt shop") includes at least one and frequently two or more primary melting furnaces of the electric arc type, at least one and frequently two or more refining facility stations, and an overall excess capacity in order that a continuous supply of molten steel can be provided to the caster. If the rolling facility downstream of the caster is designed to operate with only a standard width (and thickness) of casting--in other words, if the production rate of steel output per hour is constant--then it is a fairly simple task to design a melt shop that will supply just enough steel to keep the caster fully supplied (with a slight overcapacity to provide a margin of error). However, most steel mills are required to produce cast slabs of variable widths--in a contemporary facility, typically anywhere from 4 ft. wide to 10 ft. wide or wider. This requirement presents the problem that if the melt shop is designed to supply a continuous supply of molten steel for a long sequence of casting (say) 10-ft.-wide castings, then it will necessarily have a large overcapacity when producing, say, 4-ft.-wide steel castings. This overcapacity is typically provided at the expense of a significantly higher capital outlay than would be needed if standard-width castings only were produced, and higher capital costs lead to higher steel prices.
As steel pricing becomes increasingly competitive, it is highly desirable to reduce the capital costs associated with any equipment used in the steel mill. The capital cost associated with melt shop furnaces and associated equipment is significantly high.
Capital cost also can be higher than necessary if the melt shop takes more plant room than necessary, especially if an inefficient layout requires more or larger peripheral or support equipment (exhaust arrangements, cranes, etc.) than would be necessary for a compact and efficient layout.
Associated with the primary melting furnace in melt shops of the type under discussion are scrap bucket delivery means, charging means, and ladle transfer means. The scrap bucket delivery means is conventionally a car movable along a trackway, the scrap bucket being carried on the car from a loading area external to the melt shop into the melt shop to a bucket unloading position in the vicinity of the primary melt furnace. The charging means includes an overhead hoist or crane--the bucket is then hoisted by the overhead crane and its scrap contents are dumped into the furnace to charge it. The ladle transfer means is typically a car movable along a trackway running from the tapping position underneath and proximate to the primary melting furnace to a holding position outside the primary melt area from which the ladle filled with molten steel may be conveyed by an overhead crane or other suitable conveyor to the refining facility.
Ladles are pre-heated by a gas-fired burner, at a ladle pre-heating station, before being passed to the primary melt furnace for filling.
It is conventional that after primary melting, ladles full of molten steel will be passed directly to one or more refining facility stations for metallurgical treatment and passed thereafter to the caster. The term "refining facility" is used herein to refer to what in the industry is usually called an "LMF" or "ladle metallurgical facility (furnace)". The refining facility is the secondary heating facility used for adding small amounts of metallurgical agents to a ladle of molten steel, bubbling with argon gas, and stirring, as well as heating to a desired casting or holding temperature. If two such refining facility stations are used, the caster accepts ladles first from one refining station and then the other so as to obtain a continuous supply of steel. The refining may be done in two stages at the refining facility in a first stage, heating and argon bubbling may occur, and in a second stage, metallurgical agents may be added and the metal may be stirred to obtain uniform consistency. Normally, the electrode set is removed before the second stage begins.
Generally speaking, conventional primary scrap melting furnaces and refining facilities are each provided with a discrete electrode, or set of electrodes at least in the case of the refining facility, which latter typically operates on 3-phase alternating current.
Molten steel produced by the primary melt furnace is conventionally poured into a ladle that is then transported to the ladle finishing and refining facility. Direct current is unsuitable for use in the refining facility arrangement, because there normally cannot be a bottom electrode in a ladle. Typically, a trio of AC electrodes are used in a three-phase AC installation for providing electrical energy to the ladles in the refining facility.
To ease the refining requirement in the refining facility, it is desirable to provide a supply of alloying agents in bulk for introduction into the primary melting furnaces. This enables the primary melt to attain roughly the metallurgical composition desired for the melt. At the refining stage, trim alloys may be supplied in smaller quantities to bring the final composition of the melt to that desired.
Continuous casting cannot be carried on indefinitely--there is the necessity of shutting down the melt shop and the caster from time-to-time in dependence upon the satisfaction of a series of orders for varying quantities of product of various dimensions, and to perform maintenance and repair operations. As far as possible, however, it is desirable to be able to conduct maintenance and repair operations without any more down-time than is necessary. It is desirable to design the melt shop to be able to provide a continuous supply of molten steel to the caster for as long as required to fulfil the order at hand, or until failure of the submerged entry nozzle from the tundish into the caster mold. This may require several hours of continuous casting, and the melt shop must be able to generate the required supply.
Furthermore, energy consumption in the melt shop tends to be significantly higher than energy consumption in any other part of a steel mill. The cost of electrical energy depends not only upon the average amount of energy consumed over a period of time, but also upon the peak energy load required from time-to-time. In a typical melt shop for melting scrap, both electricity and natural gas are consumed. Electricity is consumed for the primary melting furnaces and the refining facility, and, of course, for associated blowing and pumping equipment. Natural gas is consumed to provide auxiliary heating of scrap in the primary melt furnace to provide pre-heating of ladles etc. Again, to the extent that energy expense can be reduced, the output of a steel mill can be priced competitively.
In order that a conventional melt shop be designed to match the output of the primary melting furnace(s) to the output of the ladle metallurgical refining facility, consideration has to be given to the differing lengths of time during which each type of equipment operates to perform its intended function. It will be found that the time required to perform the primary melt for a given tonnage of steel provided to fill a ladle, will exceed by a considerable margin the time required to perform the metallurgical refining, heating of the ladle contents to casting temperature, and superheating the ladle if need be. This means that there tends to be an inherent imbalance between the output of the primary melting furnaces and the refining facility that must be accommodated to permit an appropriate continuous supply of molten metal to be provided to the caster for continuous casting. One way of dealing with the imbalance is to provide a higher primary melt capacity than refining facility capacity so that the total capacity of the primary melting furnaces in tons per hour is at least approximately matched to the total capacity of the refining facility in tons per hour. This approach to melt shop design is satisfactory from the point of view of balancing the output of both the primary furnaces and refining facility, but if widely differing slab widths must be produced in the mill, the excess capacity either in the number of furnaces or in the designed tonnage capacity of furnaces adds significantly to the capital cost of constructing the melt shop.